Switching is an essential operation in communication networks. Switching is also a basic operation in digital computers and signal processing systems. The current rapid development of high-data-rate fiber-optic communications systems has created a need for high capacity repeaters and terminal systems for processing optical signals, and therefore, a need for high-speed photonic switches. Similarly, the potential for optical computing can optimally be realized if large arrays of fast photonic gates, switches, and memory elements are developed. Currently, much switching is done using an incoming datastream transmitted by fiber optics, a component to translate the data from light to electrical signals, an electrical switch, a component to translate the data from electrical signals to light, and an outgoing fiber optic cable.
When a pulse of light travels in a linear dispersive medium its length increases due to group-velocity dispersion. Depending on the intensity of the pulse and the material properties of the medium, nonlinear effects on the pulse shape, called self-phase modulation, can also be significant. Nonlinear effects are characterized by a nonlinear relationship between the polarization density and the electric field; an example is the Kerr effect. The interplay between self-phase modulation and group-velocity dispersion can therefore result in an overall pulse spreading or pulse compression, depending on the magnitudes and signs of these two effects.
Under certain conditions, an optical pulse of prescribed shape and intensity can travel in a nonlinear dispersive medium without altering its shape, as if it were traveling in an ideal linear non-dispersive medium. This occurs when the group-velocity dispersion fully compensates for the effect of self-phase modulation. Such pulse-like stationary waves are called solitary waves. Optical solitons are special solitary waves that are orthogonal, in the sense that when two of these waves cross one another in the medium, their intensity profiles are not altered, and only phase shifts are imparted as a result of the interaction, so that each wave continues to travel as an independent entity.
At a certain level of intensity and for certain pulse profiles, the effects of self-phase modulation and group-velocity dispersion are balanced so that a stable pulse, the soliton, travels without spread. The mathematical analysis of this phenomenon has so far been based on approximate solutions of Maxwell's nonlinear wave equations.
As used herein, the term soliton refers to generic solutions describing pulses that propagate without substantial change, and may be temporal or spatial. Spatial solitons are monochromatic, self-guided beams that are localized spatially in the transverse plane. They travel in a nonlinear medium without altering their spatial distribution, as a result of the balance between diffraction and self-phase modulation. Spatial solitons are the transverse analogs of temporal or longitudinal solitons. Optical pulses including solitons, may be used for photonic switching and computing.
As used herein, a switch is a device that establishes and releases connections among transmission paths, such as in communication or signal-processing systems. A control unit processes the commands for connections and sends a control signal to operate the switch in the desired manner.
A switch is generally characterized by the following parameters:
Size: number of input and output lines.
Directions: whether data can be transferred in one or more directions.
Switching time: time necessary for the switch to be reconfigured from one state to another.
Propagation delay time: time taken by the signal to cross the switch.
Throughput: maximum data rate that can flow through the switch when it is connected.
Switching energy: energy needed to activate and deactivate the switch.
Power dissipation: energy dissipated per second in the process of switching.
Insertion loss: drop in the signal power introduced by the connection.
Crosstalk: undesired power leakage to other lines.
Optical signals may be switched by the use of electrical, acoustic, and magnetic switches. For instance, in electro-optic switches, the optical signals are converted into electrical signals using photodetectors, switched electronically, and then converted back into light using LEDs or lasers. These optical/electrical conversions introduce unnecessary time delays and power loss, in addition to the loss of the optical phase caused by the process of detection. Therefore, direct optical switching is clearly preferable to non-optical switching.
In an all-optical (or opto-optic) switch, light controls light with the help of a nonlinear optical material. Nonlinear optical effects may be direct or indirect, and may be used to make all-optical switching devices. All-optical switching devices have the capability of switching at much higher rates than non-optical switching devices.
Currently, there exist a number of all-optical switching devices, including the birefringent-fiber polarization switch, the optical-fiber Kerr gate, the two-core-fiber nonlinear directional coupler, the birefringent single-core-fiber, the nonlinear fiber-loop mirror, the soliton dragging logic gate, the bistable nonlinear optical switching device, the spatial soliton beam switch in a planar waveguide, the nonlinear polarization switch in a semiconductor waveguide including a multiquantum well waveguide, the semiconductor interferometer switch, the nonlinear Bragg semiconductor waveguide switch, and the bistable optical switch.
Spatial and temporal solitons have been produced in the laboratory and used for all-optical switching. The power requirements for an optical soliton decrease as the strength of the nonlinear index of refraction increases. Therefore, the use of highly nonlinear glasses is preferable because they have larger nonlinear indices of refraction, and will significantly reduce the power requirements for the solitons.
In a nonlinear optical material, temporal soliton pulses are confined in the direction transverse to propagation by propagating in a fiber. A more maneuverable temporal soliton would be able to move in a transverse direction, such as in a planar slab waveguide. Such special types of solitons are referred to as “light bullets”. Light bullets are essentially pulses of light which, when propagating in a nonlinear material, maintain their shapes under the effect of diffraction (spreading transverse to the direction of propagation), dispersion (spreading in the direction of propagation), and nonlinearity.
However, light bullets have so far only been studied theoretically, and have not yet been produced in a laboratory. Additionally, until recently, light bullets were believed to be unstable, unless the material is saturable. The analysis for this conclusion is based on the nonlinear Schrodinger equations, which, in turn, are an approximation of Maxwell's equations. In essence, the analysis determined that light pulses will collapse. However, this analysis resorted to an approximation which neglects higher order terms in resolving Maxwell's equations, and did not take into account factors that limit the collapse, such as higher order dispersion.
A computer simulation that uses the exact Maxwell's equations without any approximation shows that light bullets are in fact stable, and that there is no need for saturating the material to obtain stability. This simulation also indicates that light bullets can deflect each others' travel paths upon collision. These light bullets will be on the order of 25 to 250 femtoseconds in duration, where one femtosecond is 10−15 seconds.
Most of the existing or previously proposed all-optical switching devices do not use or propose the use of light bullets in planar slab waveguides made from commercially available nonlinear optical glass. Prior devices often are relatively large physically or use relatively large optical pulses, as compared to the proposed device. In some of those prior devices, such as the two-core-fiber nonlinear directional coupler, the light pulses interact relatively weakly through evanescent waves. The spatial soliton devices suffer from the effects of dispersion on the pulses and the temporal soliton devices are confined to fibers and hence do not have the maneuverability of pulses in waveguides. Prior devices often do not use light bullets, which are extremely small, maneuverable and do not degrade on propagation, (i.e., are self-sustainable).
FIG. 1 illustrates a prior art embodiment of an optical switch. In particular, FIG. 1 illustrates a four-channel all-optical switching device 10. The switch 10 includes a single planar, rectangularly shaped slab waveguide 12 and a plurality of channels 14, 15, 16 and 17 that integrally depend from the waveguide 12 to provide input and output paths for the switch 10. The switch 10 may be made from highly nonlinear optical materials, including highly nonlinear optical glasses, semiconductor crystals and/or multiple quantum well semiconductor materials, and uses stable light bullets 20, 21 as optical pulses to switch each others' direction of propagation.
In this embodiment, the waveguide 12 is rectangularly shaped, and has a length “L” of about 1 cm, a width “W” of about 950 μm, and a thickness of about 2 μm. Selected candidate materials for use in the switch 10 include lead-bismuth-gallate glass, and named RN glass. The nonlinear effect is due to the product of the nonlinear index of refraction (n2) times the intensity of the optical pulse. Therefore, as the nonlinear index of refraction (n2) is increased, the power requirement can be decreased, since the power requirement is inversely proportional to the nonlinear index of refraction (n2).
Some semiconductor crystals, such as GaAs and InP, have larger nonlinear indices of refraction than nonlinear glass, and as such, they may be selected as appropriate materials for the optical switch 10. Other semiconductor crystals, such as the wide-bandgap material GaN, will allow shorter operating optical wavelengths than narrower bandgap materials, such as GaAs and InP, and as such, they may be selected as appropriate materials for the optical switch 10.
Semiconductor materials have wavelength regions, within the infrared wavelength region in which they are optically transparent. These regions depend on the individual semiconductor material. Some of these materials are III-V binary semiconductors, and other combinations of elements from groups III and V can form ternary and quaternary semiconductors. Two exemplary materials, i.e., Gallium Arsenide (GaAs) and Indium Phosphide (InP) have low losses from absorption in the transparency region.
Referring back to FIG. 1, the four channels 14 through 17 are comprised of two generally identical, elongated central channels 14, 17 that are oppositely disposed relative to the waveguide 12. The axes of symmetry of these two central channels 14, 17 coincide. Each of the central channels 14, 17 has a width “d” of about 25 μm. The other two side or lateral channels 15, 16 are disposed on either side of the waveguide 12, in a generally symmetrical relation relative to the geometrical center of the waveguide 12. Each of the exit channels 15, 16 has a width “c” of about 20 μm, and is separated from its respective adjacent central channel 14, 17, by a distance “s” of about 15 μm.
In use, a sequence of counter-propagating light bullets 20, 21 are selectively injected through the central channels 14, 17, into the waveguide 12, so that they change each others' direction of propagation, thus achieving all-optical switching. A light bullet 20 that is sent into the waveguide 12 from the central channel 14 will propagate through the waveguide 12 along a straight travel path, and will exit into the central channel 17. When two counter-propagating light bullets 20, 21 are introduced into the waveguide 12 from the central channels 14 and 17, and are axially displaced relative to each other, in the transverse direction, by the spatial width a0 of a single light bullet, these light bullets 20, 21 will collide and will deflect each other.
This interaction is an attractive one in that the light bullets 20, 21 attract each other as they pass. The result will be that the light bullets 20, 21 entering from the central channels 14 and 17, will exit into the lateral channels 16 and 15, respectively. The interaction between the light bullets 20, 21 forms the mechanism for the optical switch 10.
In the embodiment illustrated in FIG. 1, the light bullet 20 from the central channel 14 is displaced downward with respect to the light bullet 21 from the central channel 17, causing the deflection angle “b” to be approximately ½°(one half of one degree). The deflection angle is determined by the light bullet power level, the material parameters and the shape of the light bullet pulse. For instance, if the light bullet power intensity were increased, the deflection angle would increase accordingly. The deflection angle determines the length “L” of the waveguide 12.
The light bullets 20, 21 used in the switch 10 have a temporal duration of approximately 100 femtoseconds. For RN glass, the proposed wavelength of the optical carrier is about 3.5 μm, which is in the infrared wavelength range. Other proposed wavelengths of 7.85 microns for GaAs, 6.35 microns for InP, and 2.97 microns for GaN may also be used.
Although semiconductor crystals allow a decrease in the power required for the proposed optical switch 10 relative to the power requirement for RN glass, the use of quantum well semiconductor materials will allow a further significant reduction in the power requirement. The third order nonlinear susceptibility in semiconductors has been greatly increased by the use of multiple quantum well (MQW) structures in the semiconductor materials. The MQW structures that utilize AllnAs/GaInAs materials showed increases in nonlinear susceptibilities of five to six orders of magnitude greater than those associated with bound electrons in InAs and GaAs at comparable wavelengths. The MQW structures that used GaAs/AIGaAs showed comparable increases of over four orders of magnitude. These measurements were made at wavelengths of about 10.5 microns. It should be understood that various other MQW materials may be used.
InGaAs/lnP may exhibit relatively lower absorptive losses. Also, GaN/AlGaN MQW materials are another candidate material and would be able to operate at relatively shorter wavelengths. Since the power requirement for a light bullet is inversely proportional to the nonlinear susceptibility of the material the power requirement for the MQW structure may range significantly lower then the power requirements for previously described materials. Alternative nonlinear photonic glasses may be used to generate light bullets with the desired characteristic properties. Alternative semicondutor materials for use in the switch 10 to generate light bullets with the desired properties may also be used.
At a predetermined intensity, it is possible to select a group velocity dispersion of the material, which, when considered with other parameters, determines the width and the power level of the light. For each material a wavelength must be determined in order to obtain a reasonable amount of negative group velocity dispersion. This value of group velocity dispersion is used together with the nonlinear index of refraction and intensity to determine the length and width of a pulse that will form a light bullet. Also, the intensity is chosen to satisfy two requirements. First, a reasonable power level that is attainable with available power sources, such as laser equipment; and second, the intensity is such that it is sufficient to produce a strong interaction between colliding light bullets.
FIG. 1 illustrates a modification to the switch 10, in which the lateral channels 15A and 16A (shown in dashed lines) are so positioned as to form an angle “b” (i.e., equal to the deflection angle) with respect to the central axis of the waveguide 12. In such a design, the deflected light bullets 20, 21 exit the waveguide 12 through the exit lateral channels 16A and 15A, parallel to the sides of these channels.
The embodiment of FIG. 1 is known using counter-propagating light bullets traveling through channels and a waveguide all formed of the same material. Moreover, how to control the embodiment of FIG. 1 is not apparent. Therefore, there is still a great and unsatisfied need for a practical realization of an ultra-fast all-optical photonic switching device utilizing light bullets. The material used to build this device should be readily available and relatively inexpensive to manufacture, and it should further exhibit characteristic parameters that are adequate for the production and/or use of light bullets.